Do you think this was negligence (probably by an illegal) or a case of deliberate sabotage? I generally apply Occam's Razor to such things but the world became a different place starting on January 20, 2009 when a criminal regime seized power with the intent of destroying the Republic. In the fictional world of Frank Underwood, anything can happen. In the all-too-real world of hussein and his henchmen, it's clear they'll do anything to advance their wickedness.
It is not hard to miss a step. In this case I would speculate that someone reused cleaning solution rather then mixing up a new batch under the theory "Hey it is cleaning solution, how dirty can it be?"
That is why I insisted on swabs.
You get somebody new, somebody in a hurry, someone a bit absentminded and you get mistakes. Probably it is something that happened before and that time there was no deadly bug present that freezing doesn't kill. So you do it again. It is just a little short cut. Nobody gets hurt. Until the time that they do.